In the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items are supported by the drop ceiling. The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications, computer and alarm system cables. It is not uncommon for these plenums to be continuous throughout the length and width of each floor.
When a fire occurs in an area between a floor and a drop ceiling thereabove, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire story of the building and smoke can be conveyed through the plenum to adjacent areas. The fire could travel along the length of communications cables which are installed in the plenum. In the past, these cables which comprised a core of individually insulated conductors were enclosed in a jacket comprising a plastic material.
Generally, a cable in which the sheath comprises only a plastic jacket does not exhibit what are now totally acceptable flame spread and smoke evolution properties. As the jacket temperature in such a cable rises, gaseous pyrolysis products evolve, and charring of the jacket material begins. Afterwards, conductor insulation begins to decompose and char. If the jacket char retained its integrity, it could function to insulate the core, but in this cable, it is ruptured by the expanding insulation char, exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket as well as the restricted insulation char begin to pyrolize and emit flammable gases. These gases ignite and, by convection, burn beyond the area of flame impingement, propagating flame and evolving smoke.
Because of the possibility of flame spread and smole evolution, particularly when aided by flammable insulation of cables, the National Electric Code (NEC) requires that electrical cables in plenums be enclosed in metal conduits. Since rigid metal conduits are difficult to route in plenums congested with other items, a rearrangement of office telephones which in some companies has become almost an annual event, is extremely expensive. However, the code permits certain exceptions to this cost prohibitive requirement. For example, flame-resistant, low smoke producing cables without metallic conduit are permitted provided that such cables are tested and approved by an authority such as the Underwriter's Laboratories. What is need for use in buildings is a cable which is relatively inexpensive to manufacture, but which meets the NEC requirements for flame retardance and smoke evolution, and which has suitable mechanical properties such as flexibility.
In the marketplace, cable which comprises a core enclosed in a paper wrap and in a relatively thick metallic shield is available, but it is relatively inflexible and somewhat difficult to maneuver in plenums. Also, care must be taken during installation to guard against possible electrical shock which may be caused by the metallic sheath of the above-described cable engaging exposed electrical service wires or equipment in a plenum. One commercially available plastic material has been accepted as the covering material for plenum cable without the use of metal conduit, but it is relatively expensive and is difficult to process.
A plenum cable that has superior resistance to flame spread and smoke evolution, and that is attractively priced is shown in U.S. Pat. No. 4,284,842 which issued on Aug. 18, 1981 in the names of C. J. Arroyo, N. J. Cogelia and R. J. Darsey and which is incorporated by reference hereinto. It includes a reflective sheath systen which encloses a core and which comprises a layer that is made of a core wrap material and a metallic barrier or shield having longitudinal edge portions that form a seam. The metallic barrier which reflects radiant heat outwardly is covered with two translucent tapes. Each tape is wrapped helically about the core with overlapped sealed seams.
The foregoing sheath system, which depends on its reflection characteristics to keep the heat away from the core is well suited to larger pair size plenum cables. However, for smaller pair size cables such as those containing less than twenty-five pairs, the use of a metallic shield is not only expensive, but is very difficult to form about the core. Inasmuch as the metallic barrier reflects heat, manufacturing line speeds must be low enough to allow sufficient heat energy to be transferred to adhesive on the tapes to seal the seams.
What is needed and what is not provided by prior art products is a relatively inexpensive, flame retardant, smoke suppressive sheath system for a relatively small pair size plenum cable. The sought after cable desirably is easier to manufacture than presently available products and is capable of being made available in sufficient quantities to satisfy escalating demands.